This invention relates in general to fiber optic communications, and more particularly to optical transmitters and receivers for use in fiber optic communications systems.
Cable television systems typically include a headend section for receiving satellite signals and demodulating the signals to an intermediate frequency (xe2x80x9cIFxe2x80x9d) or a baseband signal. The baseband or IF signal is then modulated with radio frequency (xe2x80x9cRFxe2x80x9d) carriers, then combined and converted to an optical signal for transmission from the headend section over fiber optic cable. Optical transmitters are distributed throughout the cable system for splitting and transmitting optical signals, and optical receivers are provided for receiving the optical signals and converting them to RF signals that are further transmitted along branches of the system over coaxial cable rather than fiber optic cable. Devices known as taps are situated along the coaxial cable to split off the cable signal and direct it to the cable systems subscribers.
While cable systems have traditionally been designed in order to be one way systems, that is for information to flow from the cable headend to the subscriber""s location, changes in the cable industry have necessitated the ability for information generated at subscriber locations to flow back to the headend. Accordingly, cable systems have been recently modified from having simply a forward path, i.e., information flowing from the headend to the subscriber, to include a reverse path for allowing information from the subscriber to flow back to the headend. Examples of information that would flow in the reverse path include data relating to status monitoring of the subscriber device, subscriber pay-per-view programming selections, cable modem information, and two-way video and telephony services. Information transmitted from the headend to the subscriber is typically in the frequency range of between approximately 50 megahertz (xe2x80x9cMHzxe2x80x9d) and 750 MHz, while information being transmitted in the reverse path is typically in the frequency range from between 5 and 40 MHz.
Various factors influence the ability to accurately transmit and receive optical signals within a cable television system. As the length of fiber optic cable within a system increases, for example, signal losses also increase. Furthermore, temperature fluctuations which cause variation in the optical modulation index of the optical transmitter, can result in variation of the RF output level for the optical receiver. Signal distortions may also be caused by non-linearities in the optical transmitter laser and photodiode of the optical receiver. Finally many of the devices interposed in the forward and reverse path themselves introduce noise and other distortions into the system. Accordingly, in many instances the range of the particular system in question is limited both in terms of distance and bandwith capability.
Although these problems may be mitigated by employing expensive techniques, e.g., decreasing fiber length between optical nodes, such techniques may prohibitively increase costs to both subscribers and service providers. Accordingly, there exists a need for more reliable and accurate transmission of optical signals within a cable television system. In particular, improved optical signal reliability and accuracy in reverse path transmissions is critically needed.